Mass balance is the balance between the inputs and outputs within a system. With manure, phosphorus (P) mass balance is used to estimate the acres of land or the amount of “root zone” needed to use livestock manure nutrients. Simply, P balance equals P imported minus P exported.
Environmental risk to surface and groundwaters is increased if the amount of P brought onto the farm (e.g., from fertilizers, feed and animals) exceeds the amount of P leaving the farm (e.g., crops, animals, manure or animal products). Mass balance estimates shown in Table 1* were derived using standard diets, animal performance, and 50 pounds per acre P2O5 crop removal.
Livestock producers may consider moving manure off site, but when not feasible potential feeding strategies to reduce needed land base include:
•Diets should be formulated based on “available P.” Pigs and birds are able to absorb only part of the P in feeds. Grains for swine and poultry can vary from 14 to 50 percent available P, whereas over 90 percent of P is available in cattle and sheep feeds due to microbes in the rumen.
•Formulate rations to meet the animal’s exact P requirements for maintenance, lactation, growth and pregnancy. For lactating dairy cows, 1 gram of P for each pound of milk produced generally is sufficient and is fed when dietary P ranges from 0.32 to 0.38 percent (dry matter basis).
•Routinely complete laboratory analyses of feeds and rebalance rations.
•Cattle rations may not need P supplementation to meet the animals’ requirements. Discontinuing P supplementation may reduce land base by 25 to 50 percent (depending on the amount of oversupplementation in the original feeding program).
•If typical rations (corn silage, soybean meal, alfalfa and corn grain) contain more P than needed to meet requirements, and if land base is limiting, alternative feedstuffs should be considered. The cost of using alternative feedstuffs may be less than the cost of using common “least-cost” feeds and managing excess manure P.
•Phytase in corn-soybean meal-based diets for swine and poultry increases P availability so 25 to 35 percent less total P is required in the feed.
•Pellet and reduce the particle size of rations to increase the efficiency of P use by pigs and poultry by 5 to 10 percent. Formulate rations for specific production phases, genotypes and gender. “Phase- feeding” programs for growing swine and dairy cows can reduce P by 5 to 10 percent. ANM
Table omitted but is available upon request to editor@progressivedairy.com.
—From Michigan State University
